1. Field of the Invention
This disclosure relates generally to a linear actuated force motor that requires low power input and provides a long proportional stroke. More particularly, this disclosure relates to a technique to control local magnetic field distribution so as to provide a long proportional stroke.
2. Description of the Related Art
FIG. 1 shows a cross-sectioned view of a conventional force motor. A conventional force motor includes a shaft 1 mounted in bearings 2 that are mounted in a housing 3. An armature 4 is mounted on the shaft. Two springs 5 and 6 are mounted on the shaft with the armature located between the springs. The springs keep the armature in the neutral position when no net axial force is being exerted on the armature. The armature shaft is free to slide on the bearings in axial directions. A permanent magnet 7 is located at the periphery of the armature. Two coils 8 and 9, wound in the same direction are located on each side of the permanent magnet.
The permanent magnet produces a magnetic field Bp. When energized, the coils produce a magnetic field Bi. Since the coils are wound in the same direction the magnetic field Bi produced by the coils is in the same direction as the magnetic field Bp on one side of the permanent magnet and in the opposing direction on the other side of the permanent magnet. Thus, the resultant magnetic field on one side of the permanent magnet is Bp+Bi and on the other side of the permanent magnet is Bp−Bi. See FIG. 2. The electrical force produced on the armature is proportional to the square of the magnetic field and can be calculated as follows.F=KB2  Eqn. 1                Where F=electrical force                    B=Magnetic flux density            K=ConstantUsing equation 1, the net force on the armature of a force motor when the coils are energized can be calculated as follows:                        
                                                                        F                fm                            =                              K                ⁢                                  {                                                                                    (                                                                              B                            p                                                    +                                                      B                            i                                                                          )                                            2                                        -                                                                  (                                                                              B                            p                                                    -                                                      B                            i                                                                          )                                            2                                                        }                                                                                                                        =                                  4                  ⁢                                                                          ⁢                                      KB                    p                                    ⁢                                      B                    i                                                              ⁢                                                                                                      Eqn        .                                  ⁢        2            For a proportional solenoid wherein a coil produces a magnetic field equal to Bi, the net force on the armature can be calculated using equation 1 as follows:Fps=KBi2  Eqn. 3    Now ifBp>Bi then4Bp>>Bi ThereforeFfm>>FThus, by using a permanent magnet, for a given level of coil energization (i.e. current), the force motor produces larger net force on the armature. Therefore, for a given force requirement the force motor can be operated with lower power input compared to the proportional solenoid. If Bp is assumed to be constant in equation 2, it is clear the net force is proportional to the magnetic field produced by the coils.Ffm=CBi  Eqn. 4    where            C=4KBp, assuming Bp=constant            Since Bi is proportional to I    where I is the current supplied to the coils,            Ffm is proportional to Ii.e. the net force on the armature is proportional to the current supplied to the coils.        
However, Bp can be assumed to be constant only when the armature is in the neutral position. As the armature moves away from the neutral position, Bp changes. When the armature moves, Bp on one side of the armature increases whereas Bp on the other side of the armature decreases. This results in a dramatic increase in the net force on the armature. Thus, in a conventional force motor, the force is proportional to the stroke only within a small range of the stroke, for example 0.01 to 0.03 inches.
U.S. Pat. No. 5,787,915 describes a conventional force motor having a permanent magnet and coils. However, it does not teach any means of providing increased proportional stroke.
U.S. Pat. No. 3,900,822 (the '822 Patent) describes a conventional proportional solenoid with a conical pole piece on each side of the bobbin. When the solenoid is energized, the armature is pulled to one side and enters into the conical pole piece. The conical pole piece provides a leakage flux path and thereby reduces the increase in the net force on the armature. The proportional solenoid similar to that of the '822 Patent requires higher power input compared to the force motor of the present invention to produce the same amount of force on the armature.
The use of a conical pole piece as taught by the '822 Patent does not provide a substantial increase in proportional stroke. Additionally, when a conical pole piece is used, the proportionality and the constancy of the net force on the armature gets worse with increase in current (I) supplied to the coils or when the plunger position changes.